It is well known that the operation of rotary offset presses is based on the repulsion exerted by a dampened surface on greasy ink. As a result, such presses are necessarily fitted with a dampening system whose primary function is to deposit a uniform film of aqueous solution on the printing plate which is fitted around the plate cylinder. This film must be very thin (thickness about 1 micrometer), and it must be very uniform. The slightest non-uniformity leads to a change in the optical density of the printed signature (the uniformity of the optical density is spoiled), and this gives rise to a defect which is more or less easily visible on the printed signature. In general, the aqueous solution used includes certain additives, in particular, isopropyl alcohol, for reducing its surface tension, and/or citric acid for cleaning the plate.
There are thus various different dampening systems commonly to be found on rotary offset presses, be they sheet-fed or web-fed.
Mention may initially be made of alternating ductor dampening systems.
Systems of this type comprise firstly a metal pan roller which is partially immersed in a trough containing the dampening solution and which is rotated slowly by an independent electric motor, and secondly a pair of rollers rotating at machine speed (quickly), namely a chromium-plated metal roller constituting a dampening drum, and a rubber-covered dampening roller in contact with the plate cylinder. The dampening roller is usually fitted with a cloth sleeve to provide spare capacity. Another roller, referred to as a "ductor" roller, is mounted on a reciprocating bracket and is thus capable of making contact alternately with the pan roller and with the dampening drum, thereby transferring the film of dampening solution from one group of rollers to the other.
Such systems are being used less and less today, since the uniformity of the moistening film they provide is not perfect, and since they also require the use of a cloth sleeve on the dampening roller, which sleeve is subject to rapid wear and must be replaced periodically.
Mention may also be made of brush dampening systems.
Systems of this type serve to spray moistening solution onto the dampening vibrator by using a circular brush having the end of its bristles dipping into a trough of solution and coming into contact with fixed scrapers which deflect the bristles of the brush, thereby spraying droplets onto the dampening vibrator. The dampening solution is thus fed permanently to the pair of rollers that rotate at machine speed (the chromium-plated dampening drum and the rubber-covered dampening roller). In a variant, the circular brush rubs against a pan roller which is partially immersed in the solution, with the bristles of the brush spraying droplets as they go past.
Such systems, and other spray dampening systems, are typically not satisfactory, as the film obtained is never perfectly uniform because droplet spraying is relatively random.
In order to avoid the above-mentioned drawbacks of dampening systems using a brush or an alternating ductor, proposals have been made for so-called "film" dampening systems.
In general, film dampening systems include two groups of rollers. A first group of rollers takes solution from the dampening trough or pan and rotates at a speed which is variable but which nevertheless remains less than machine speed (the linear speed of the plate fixed on the plate cylinder). The disposition of this first group of rollers is designed so that one of the rollers in the group is covered with a uniform film of solution. A second group of rollers is organized in contact with the plate disposed on the plate cylinder and rotates at the same linear speed as the plate, enabling a dampening solution to be deposited thereon.
Solution is transferred between these two groups of rollers by sliding contact. Since all of the rollers are parallel to one another, and since they touch one another along generator lines (i.e., transverse contact zones), the film of dampening solution to be applied uniformly onto the plate is transported from roller to roller, from the pan roller in the first group to the dampening roller in the second group.
In film dampening systems, whenever two rollers touch along a generator line, two situations may arise:
either both rollers rotate with substantially the same linear speed, the linear speeds being related by a constant ratio, in which case the rollers can be pressed firmly against each other firmly; or else
the two rollers rotate at linear speeds which are very different, in which case they are pressed more gently against each other so as to allow the rollers to slip relative to each other.
In general, the film dampening systems presently available on the market can be divided into two broad categories, namely three-roller "emulsion" dampening systems, and four-roller "independent" dampening systems.
Three-roller emulsion type film dampening systems comprise in succession: a pan roller covered with an elastomer and partially immersed in solution; a chromium-plated metering roller; and an elastomer-covered dampening form roller. In this system, the thickening of the dampening film on the metering roller is determined by the dampening solution passing between the elastomer-covered pan roller and the chromium-plated metering roller which are pressed against each other. A variable speed electric motor drives the pan roller and the chromium-plated metering roller which is coupled thereto by gearing. A dampening form roller is in contact with the plate of the plate cylinder and with a vibrated ink drum rotating at machine speed. The dampening form roller of the emulsion dampening system is thus the only one of the three rollers of the system to be driven at machine speed, with slipping contact being established directly on the surface of the form roller making contact with the metering roller.
Emulsion film dampening systems of the above-described type are illustrated, for example, in the following French patents: No. 1 374 410, No. 1 547 536, No. 2 058 506, No. 2 196 249 (which corresponds to U.S. Pat. No. 3,986,452), No. 2 274 366 (which corresponds to U.S. Pat. No. 3,937,141), and No. 2 448 978.
These film dampening systems, which are generally described below with reference to FIG. 1 of the accompanying drawings, have the advantage of limiting gap transfers by spreading out the excess water remaining on the form roller after passing over the plate gap, by virtue of its slipping contact with the metering roller, but they nevertheless suffer from the drawback of using high percentages of isopropyl alcohol for reducing surface tension (with the water-ink emulsion being made prior to contact on the plate).
Four-roller independent type film dampening systems comprise in succession: a partially immersed pan roller which is chromium-plated (or which is fitted with a hydrophilic material); an elastomer-covered metering roller; a dampening drum which is vibrated (i.e. a dampening "vibrator") and which is chromium-plated (or covered with a hydrophilic material); and an elastomer-covered dampening form roller. In this system, the thickness of the dampening film is defined by the pressure between the chromium-plated pan roller and the elastomer-covered metering roller. As in the emulsion systems, a variable speed electric motor drives the pan roller and the metering roller which is coupled thereto by gearing, whereas the dampening roller is driven at machine speed. In this type of system, the slipping contact therefore occurs between the elastomer-covered metering roller and the chromium-plated dampening roller (with the pressure between the metering roller and the dampening roller then being deliberately adjusted to "light"). The dampening form roller of the independent dampening system thus remains in contact with the dampening roller without slipping, and slipping takes place at the surface of the dampening roller between the dampening roller and the metering roller.
Such independent film dampening systems of the above-described type are illustrated, for example, in French patents No. 1 491 977 and No. 2 211 348 (which correspond to U.K. Patent No. 1,163,267 and U.S. Pat. No. 3,842,735, respectively).
Such film dampening systems, which are described generally below with reference to FIGS. 2 and 3 of the accompanying drawings, have the advantage of being capable of using relatively weak concentrations of alcohol, particularly because of the vibration of the dampening vibrator. However, they nevertheless suffer from the drawback of getting rid of gap transfers poorly (the excess water remaining on the dampening roller upon passing over the plate gap is squeezed at the area of contact between the roller and the dampening vibrator roller, but it is not sufficiently reduced since the contact is not a slipping contact).
Thus, each of the two above-mentioned film dampening systems has both advantages and drawbacks, and a user must make an initial choice as to which dampening system to use.
This can be somewhat restrictive insofar as the type of printed signature that is to be used and its coverage ratio, and also the types of defects that it is desired to eliminate in preference to others are not always definitive. Accordingly, it would be extremely advantageous to be able to use one dampening mode or the other depending on the circumstances.
Unfortunately, prior systems are not easily modified to switch from one mode to the other, and some simply cannot be transformed between modes.
An object of the present invention, therefore, is to propose a film dampening system which is dual-mode and convertible, capable of operating equally well in either of two different dampening modes, comprising an emulsion type mode and a four-roller independent type mode, thereby making it possible to use each mode to the best advantage depending on any given situation.
Another object of the present invention is to provide a film-dampening system which is easily converted, i.e., in which switching from one mode to another does not require long and difficult disassembly and adjustment operations.
Yet another object of the present invention is to provide a convertible dual-mode dampening system capable of being fitted equally well to sheet-fed and to web-fed rotary offset presses.
Other objects and advantages of the present invention will become apparent in view of the following detailed description and accompanying drawings.